1. Field of the Invention
The present invention relates to medical X-ray imaging, and more particularly relates to collimator sensitive histogram generation for use in the automatic adjustment of window level settings in medical X-ray imaging.
2. Description of the Related Art
Conventional X-ray imaging systems for digital radiography are used for various imaging applications including without limitation anatomical background imaging and angiographic imaging to highlight a patient's vasculature using such techniques as digital subtraction angiography (DSA) and live fluoroscopy roadmapping. X-ray imaging systems include an X-ray source and X-ray capture device such as a digital flat panel detector to convert the X-ray energy striking the detector into a latent image frame. That is, the X-ray beam or radiation strikes a CCD or flat panel detector, where the captured radiation image is converted into a digital signal, comprising rows and columns of pixels. The X-ray beam is adjusted and shaped to facilitate image acquisition, whereafter the digital signal data are manipulated to better visualize the image produced.
X-ray imaging systems may include various displays, panels, consoles, workstations, etc., with user interfaces such as keyboards, switches, dials, trackballs, joysticks, etc., that enable an operator to control operations such as image contrast, brightness, image blur and noise in the produced image. X-ray systems include various mechanisms for preventing direct exposure from the X-ray beam as well as from scattered X-rays from reaching the flat panel detector. The mechanisms also include X-ray collimators or beam-limiting devices, which adjust the shape of the radiated X-ray beam to an extent necessary for imaging patient anatomy within a desired field of view (FOV). For example, a collimator may be adjusted during the examination for each image taken to optimally cover or mask part of the X-ray beam FOV in which no body part, or non-relevant body parts are located, i.e., outside the FOV. Because manually setting the collimator, for example, at each station used to carry out an angiographic study of leg vasculature for a mask run, and saving the settings for actual image acquisition is cumbersome and time consuming, automatic collimator adjustment functions have been developed. U.S. Pat. No. 6,055,295, commonly owned, discloses a system and method for automatically setting the collimator of an X-ray imaging system at the time of image acquisition.
As mentioned, the raw image data striking the detector is arranged in and transferred from the detector in a form of pixels. Pixels may have digital intensity values limited by the system contrast ability and the digital size of the pixel. For example, a 12-bit pixel may have a value from 0 to 4095. A look-up table (LUT) is used to map pixel values to one of the shades of grey in the displayed image. The digital image comprising a frame or set of pixels is processed by mapping the digital gray values into specific densities or luminances for a specified display means, e.g., a CRT display. Difficulties arise, however, in attempting to display the full acquisition dynamic range, such as the aforementioned image blackening found in film-based systems. Collimators may be used to block (by limiting the X-ray beam) the area outside the FOV. This process normally results in black borders comprising the shielded pixels surrounding the field in the viewed image. Radio-opaque elements and substances such as a contrast medium used in angiographic applications also appear black in an acquired image frame. Without background removal, background may affect post-acquisition processing and decrease contrast levels and therefore image quality.
Techniques such as window width (contrast), window level adjusting (brightness) and edge enhancement (unsharp masking), and background removal have developed to improve image display. Background removal processes improve contrast in the displayed image. Window level adjustment processes may be controlled manually at a workstation or automatically in an imaging chain background process to improve viewed image quality. U.S. Pat. No. 6,106,152, commonly owned, discloses an X-ray imaging system with an X-ray source that includes an X-ray tube and collimator to limit or adjust the radiated X-ray beams. A test exposure may be acquired with the system to adjust the collimators or position the X-ray beams in relation to a digital detector, such as a flat panel detector, focusing the FOV. An image of the outer contours of the patient may be obtained and processed to have sufficient contrast for viewing on a display.
Processing shifts of the upper or lower edge of the acquired image gray-scale level changes the window range, carried out by first determining a minimum and maximum of a global image histogram, automatically, or based on manual inputs to adjust a windowed image. A histogram is a gray scale value distribution showing the frequency of occurrence of each gray level value in the image. U.S. Pat. No. 6,127,669 discloses window and level control based on histogram analysis. In such histogram based level adjustment systems and processes, the minimum and maximum levels within the histogram define the window range, e.g., their average value. Conventionally, the window range and window level are used to generate the default look-up table, or LUT. Background removal is based on an assumption that the gray-scale values of foreground (anatomical structure in the field of view) and background (outside patient boundary) are distinguishable by use of the histogram.
Pixels outside the exposed viewing area can cause histograms used for the automatic window adjustment to be skewed, resulting in incorrect image display because known histograms are computed over the entire image. Accordingly, known histogram calculations attempt to exclude acquired image pixels that are presumed to be in the raw image areas under the collimator. That is, conventional histogram calculations are based on a presumption that those pixels of collimated image areas contain the darkest pixels of the image. Preconfigured numbers of pixels are therefore excluded from the low and high ends of the histogram before window level settings are calculated. But where the collimator area is significant, or if there is image processing in the imaging chain before histogram calculation such that the collimated area is not the darkest area of the image, or if the pixels under the collimator are undefined by the data source, the histogram becomes skewed, affecting image quality.
It would be desirable, therefore, to the skilled artisan and clinician alike to have use of a system and method that overcomes the shortcomings of prior art histogram calculation and modification, excludes pixels known to be under a collimated area from the histogram calculations, particularly in angiographic imaging. To that end, the present invention provides a system and method that overcomes the shortcomings of prior art histogram calculation and modification, which excludes pixels known to be under a collimated area from the histogram for use in various processes, including window level calculations.